Magelona papillicornis is not a true synonym. Magelona papillicornis has not changed its name and still exists off the coasts of Brazil.
However, Magelona mirabilis includes the north east Atlantic animals that were once called Magelona papillicornis (M. Kendall, pers. comm.).
See Jones (1977) for further taxonomic information.

For detailed notes on the identification of European Magelona sp., see Fiege et al. (2000).

Abundance
Occurs at high densities where environmental conditions are suitable. For example, Kuhl (1972) reported Magelona papillicormis at densities of 279 individuals per 0.1 m² on sandy muddy ground in the Elbe Estuary.FeedingMagelona mirabilis feeds by gathering organic material from the sediment surface with its palps. When feeding on poorly sorted material, selectivity may be shown in that magelonids prefer to handle larger particles. Small crustaceans may also be taken as prey, for example, the mucous on the palps may trap a few harpacticoids although this is likely to be incidental (M. Kendall, pers. comm.). In well sorted sand, selectivity may be absent as particles with a high organic content have already been concentrated by other means (Fauchald & Jumars, 1979).

Reproductive data concerning Magelona mirabilis is scarce (Fiege et al., 2000). The data that is available suggests that the reproductive period in Magelona mirabilis varies with geographic location and the breeding season of many polychaetes is known to vary with latitude. Fiege et al. (2000) reported males with sperm masses in St. Andrews, Scotland, in March and females with eggs in Berwick-upon-Tweed in March whilst egg bearing females in Lancieux, France, were recorded in May. It is generally agreed that Magelona mirabilis displays characteristics typical of an r-selected species, i.e. rapid reproduction, short life span and high dispersal potential (Krönke, 1990; Niermann et al., 1990), and is characteristic of shallow, disturbed, non-successional habitats (M. Kendall, pers. comm.).